Method of making a low density coating for an electron discharge device



United States Patent METHOD OF MAKING A LOW DENSITY COAT- ING FOR AN ELECTRON DISCHARGE DEVICE Theodore W. Blickwedel and Robert L. Lambert, Emporium, Pa., assigu'ors, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware No Drawing. Filed Dec. 26, 1957, Ser. No. 705,099

4 Claims. (Cl. 117-221) This invention relates to electron discharge devices and more particularly to electron emissive and insulator coatings of the type adapted to be used in such devices.

Various electrodes of an electron discharge device or tube may have deposited thereon an electron emissive material or an insulating coating. For instance, the cathodes of most tubes comprise a metallic sleeve or wire having a coating of alkaline earth carbonates for providing the source of electrons for the tube. The insulating coating may comprise materials such as the refractory oxides for use on certain electrode supports and on the cathode heater wire.

Both of the coatings mentioned above generally have a density in excess of two grams per cubic centimeter. This density is undesirably high for many applications, and it very often causes problems during tube fabrication. For instance, 'When the emissive coating density is high, it is difficult to process the coating properly to obtain the optimum electron emission characteristics. When the density of the insulating coating is high, the processing is more critical, and, when this coating is used on heaters, the warm-up time of the heater is materially and undesirably increased. Accordingly, it is an object of the invention to decrease the aforementioned difliculties and to facilitate processing of coatings of the type adapted to be employed in electron tubes.

A further object is to improve the emission characteristics of cathode coatings.

Another object is the provision of means for increasing the thickness of insulator coatings without increasing the mass thereof.

A still further object is to provide means for facilitating fast warm-up time for electron tube heaters.

A still further object is to produce low density coatings adapted to be employed with the electrodes of an electron tube.

The foregoing objects are achieved in one aspect of the invention by the provision of a coating suspension which may comprise either the insulating or electron emissive materials, an organic liquid binder, and a particulate heat gasifiable additive material insoluble in said binder or one which will crystallize on the metallic electrode. These coatings may be formed as a suspension for electrophoretic, spray or coating strip-transfer applications to the intended electrode. The coating is heated after deposition to gasity the additive material, thereby lowering the density of the coating. The amount of additive material used determines the density of the processed coating.

The electron emissive material utilized in one aspect of the invention may comprise alkaline earth metal salts such as nitrates or carbonates of barium, strontium and calcium. These compounds may be employed as double or triple carbonates or nitrates in given proportions to provide the electron emission characteristics desired.

A coating suspension adapted to produce an electron emissive cathode coating may comprise a triple carbonate 2 of, for instance, 56% BaCO 40% SrCO and 4% CaCO by weight. This mixture may be added to a liquid organic volatile binder material such as nitrocellulose or methacryolate lacquers dissolved in solvents such as ethyl or butyl acetate, toluene, benzolene, xylene, etc.

In accordance with one aspect of the invention, a par ticulate low temperature gasifying, subliming or decomposable material which may be insoluble in the binder or of a nature which will crystallize on the metallic electrode is also added to the binder and triple carbonate solution. Such an additive may comprise inorganic materials like arnmonum bicarbonate, ammonium carbamate, ammonium acid carbonate, ammonium acetate, ammonium formate in addition to organic compounds like angelic acid, crotonic acid and piperazine.

A suspension for producing insulating coatings may comprise the binder and additive material described above with the inclusion of an insulating material like the refractory oxides or silicates of aluminum, beryllium, titanium, and zirconium.

An example of an electron emissive suspension which has provided excellent results and produces acoating having a density of approximately .6 gram per cubic centimeter is given below:

Percent Material Amount y Weight Triple carbonate grams 8. 41 Ammonium bicarbonate- 25 grams 2. 60-80 second nitrocellulose (dry) 1.5 grams 0. l7 Amyl acetate 900 ml 88. 62

It has been found that the amount of emissive material in the suspension may be varied over wide limits in-accordance with the electron emission characteristics desired in the tube.

An example of an insulating material suspension which has provided excellent results and produces a coating having a density of approximately 1.0 gram per cubic centimeter is given below:

Percent Material Amount by Weight Aluminum oxide powder 50 grams 18. 04 Ammonium bicarbonate 50 grams 18.04 9001,200 second nitrocellulose (dry)- 1.35 gr ms 0. 49 Amyl ac 200 ml 63. 43

whereas a cathode sleeve may be more easily coated by either the spray or strip-transfer processes. In view of these various coating deposition techniques, the amount of liquid binder or lacquer in the suspension is generally adjusted to afford easy application in the technique selected.

After the electron emissive or insulating coatings have been deposited upon their electrodes, the coating is heated by any conventional means to cause the additive material to gasify, thereby forming a low density porous type coating. These electrodes are subsequently mounted Within the tubes, which are then processed by heating at elevated temperatures during an exhaust operation to remove gases such as the organic binder and to form the insulating material, 'e.g. fused aluminum oxide, and the ernissive materialfegl oxides of barium, strontium and calcium.

d It has been found preferable to use an additive material which decomposes by gasification and/or by cry'stallization on the metallic electrode at a temperature below 300 C. so that the coating may be easily processed to acquire a low density prior to insertion into the tube envelope. This affords use of a less critical tube processing schedule. The ammonium bicarbonate may be removed at temperatures as low as 60 degrees centigrade. If desired, the initial heating step used to gasify the additive material need not be employed. This mate'rial may then be removed during regular tube processing. However, care must be exercised in this instance to prevent raising the coating temperature at'an excessively high rate since such action may cause the gasifying material to rupture the coating.

It has been found that the addition of percent by weight of ammonium bicarbonate (based on total solids) to the alkaline earth carbonate suspension reduces the density after heating to 1.10 grams per cubic centimeter and that 25 percent of ammonium bicarbonate reduces the density to .60 gram per cubic centimeter. The density of such a coating without the additive material is generally about 2.0 grams per cubiccentimeter.

The addition of 10 percent by weight of ammonium bicarbonate (based on total solids) to the alumininn oxide suspension reduces the density of the. coating after heating to 1.7 grams per cubic centimeter and 50 percent of ammonium bicarbonate reduces the density to 1.0 gram per cubic centimeter. The density of such a coating without the additive material is also generally about 2.0 grams per cubic centimeter.

Although several embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

1. A method of forming a low density cathode for an electron discharge device comprising the steps of depositing upon a metallic base member a layer of a composition including an alkaline earth metal salt, an organic binder and an inorganic compound in'solublein said binder selected from the group consisting of ammonium bicarbonate, ammonium carbamate, ammonium acid carbonate, ammonium acetate and ammonium formate, and heating said composition urider 300 C. to substantially decompose said compound.

' 2. A method of forming a low density cathode for an electron discharge device comprising the steps of depositing upon a metallic base member a layer of a composition includingan alkaline earth'metal salt, an organic binder and an organic compound selected from the group consisting of angelic acid, crotonic acid, and piperazine, and heating said composition under 300 C. to substantially decompose said compound.

3. A method of forming a low density insulating coating upon an electrode of an electron discharge device comprising the steps of depositing upon the electrode a layer of a composition including a refractory oxide, an organic binder and an inorganic compound insoluble in said binder selected from the group consisting of ammonium bicarbonate, ammonium carbamate, ammonium acid carbonate, ammonium acetate and ammonium formate, and heating said composition under 300 C. to substantially decompose said compound.

4. 'A method of forming a low density insulating coating upon an electrode of an electron discharge, device comprising the steps of depositing upon the electrode a layer of a composition including a refractory oxide, an organic binder and an organic compound selected from the group consisting of angelic acid, 'crotonic acid," and piperazine, and heating 'said composition under 300 C. to substantially remove said compound Anderson Mar. 8,1955 

1. A METHOD OF FORMING A LOW DENSITY CATHODE FOR AN ELECTRON DISCHARGE DEVICE COMPRISING THE STEPS OF DEPOSITING UPON A METALLIC BASE MEMBER A LAYER OF A COMPOSITION INCLUDING AN ALKALINE EARTH METAL SALT, AN ORGANIC BINDER AND AN INORGANIC COMPOUND INSOLUBLE IN SAID BINDER SELECTED FROM THE GROUP CONSISTING OF AMMONIUM BICARBONATE, AMMONIUM CARBOMATE, AMMONIUM ACID CARBONATE, AMMONIUM ACETATE AND AMMONIUM FORMATE, AND HEATING SAID COMPOSITION UNDER 300*C. TO SUBSTANTIALLY DECOMPOSE SAID COMPOUND. 